1. Field of the Invention
The present invention relates to a spin valve transistor with a self-pinned antiparallel (AP) pinned layer structure wherein first and second AP pinned layers of the AP pinned layer structure comprise different materials for self-biasing the AP pinned layer structure.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. When the disk is not rotating the actuator arm parks the suspension arm and slider on a ramp. When the disk rotates and the slider is positioned by the actuator arm above the disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. The ABS is an exposed surface of the slider and the write and read heads that face the rotating disk. When the slider rides on the air bearing, the actuator arm positions the write and read heads over the selected circular tracks on the rotating disk where signal fields are written and read by the write and read heads. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a current perpendicular to the planes (CPP) sensor, such as a magnetic tunnel junction (MTJ) sensor, for sensing the magnetic field signals from the rotating magnetic disk. The MTJ sensor includes an insulative tunneling or barrier spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to the air bearing surface (ABS). The MTJ sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the MTJ sensor for conducting a tunneling current therethrough. The tunneling current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to or, otherwise stated, conducted in the planes of the major thin film planes (CIP) of the spin valve sensor. Another type of CPP sensor employs a nonmagnetic electrically conductive spacer layer instead of an insulation material spacer layer. In both types, a magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, occurs when the tunneling current is conducted through the sensor without magnetic signal fields from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the CPP sensor to the tunneling current (IT) is at a minimum and when the magnetic moments are antiparallel the resistance of the CPP sensor to the tunneling current is at a maximum. Changes in resistance of the sensor is a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the tunneling current (IT) is conducted through the sensor, resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the MTJ sensor at minimum resistance. The dr/R of a CPP sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
Another type of sensor is a spin valve transistor (SVT) wherein the amount of current conducted therethrough is detected as playback signals. The SVT comprises an emitter, a collector and a base therebetween wherein the base comprises at least a portion of a CPP type of sensor. In an exemplary embodiment the CPP sensor comprises a free layer structure, a pinned layer structure and a nonmagnetic spacer layer therebetween. Input and output leads are connected to the emitter and collector respectively and are commonly connected to the base such as to the free layer. Hot electrons are injected by the emitter into the base and a portion of these hot electrons that are not scattered within the layers or at the interfaces of the layers are conducted through the base to the collector. The hot electrons which are scattered flow back to the emitter via a common base electrode. Only electrons that tunnel through an insulation layer, such as Al2O3, at the emitter are injected into the base. These are hot electrons which have an energy level above Fermi levels of the layers within the base. Only electrons above the Fermi levels contribute to conduction of electrons through the base. The mean free path of these hot electrons is long compared to lower energy electrons and they have a mean free path which is greater than the thickness of the spacer layer between the free and pinned layers.
Hot electrons, like all electrons, have a spin orientation. Spin up electrons are oriented in the same direction as the magnetic moment of the magnetized material and spin down electrons are antiparallel thereto. Because the spin up electrons are the majority of the electrons in the material the magnetic moment is in the same direction as the orientation of the spin up electrons. Accordingly, the spin up electrons are referred to as majority electrons and the spin down electrons are referred to as minority electrons. The conduction of the hot electrons through the base is spin-dependent upon the relative orientation of the magnetic moments of the free and pinned layers. When the magnetic moments of these layers are parallel the conduction of hot electrons through the spacer layer is high and when the magnetic moments of these layers are antiparallel the conduction of hot electrons through the spacer layer is low. The conductivity of hot electrons through the pinned layer structure is also important, which will be discussed in more detail hereinbelow.
With the ordinary CPP sensor, discussed hereinabove, a high magnetoresistive coefficient dr/R is sought because these resistance changes are manifested as potential changes which are processed as playback signals. The magnetoresistive coefficient dr/R of a CPP may be as high as 40% whereas the magnetoresistive coefficient dr/R of a SVT can easily exceed 200%. Instead of detecting potential changes as in the ordinary CPP, current changes between the base and collector are detected and processed as playback signals in a SVT. Unfortunately, a SVT can have a very low transfer coefficient which is the collector current to the emitter current (IC/IE). Accordingly, it is desirable to increase this coefficient in order to improve the read capability of a read head incorporating the SVT.
In commonly assigned U.S. Pat. No. 6,480,365, hereinafter referred to as Gill patent, and U.S. Pat. No. 6,501,143, hereinafter referred to as the Sato patent, spin valve transistors are discussed. Both the Gill patent and the Sato patent are incorporated by reference in their entireties herein. In the Gill patent a pinned layer is pinned by a pinning layer, such as platinum manganese (PtMn). The Sato patent does not describe the scheme for pinning the pinned layer structure. It is important that the entire base, including the pinned layer structure, be as thin as possible so as to minimize scattering of hot electrons. The platinum manganese (PtMn) pinning layer in the Gill patent is typically 150 Å thick which is also true for the Sato patent if a pinning layer pins his pinned layer.